Fifty-two miners were working underground at the Zhujiadian Coal Mine in Luliang, in the northern province of Shanxi, when the explosion took place at about 4:20 p.m. Wednesday, Xinhua quoted provincial authorities as saying.

It said "intensive rescue efforts" were under way and that the fate of the 44 trapped workers was unclear.

China's mines are among the world's deadliest. They often lack even basic safety equipment, and miners are usually too poorly trained to understand the risks. The government has made frequent promises to improve the situation and has closed thousands of mines around the country in the past year.

More than 3,500 miners have been killed this year in explosions, floods and other mishaps in Chinese mines.